To meet National Electrical Code requirements for electrical safety in hazardous locations as outlined in NFPA-70 Article 500 and 501, the manufacturers of ignition systems for internal combustion engines have developed and are producing low hazard shielded ignition systems suitable for use in Class 1, Division 2, Group D hazard locations. These systems, under normal conditions, cannot release sufficient energy into the surrounding atmosphere to cause ignition of a flammable mixture of gas and air.
A common ignition system used in hazardous locations is a capacitor discharge ignition which includes: a mechanically driven alternator, a rectifying means, a power storage capacitor, a switching means, a step-up ignition transformer, and a spark plug with an ignition transformer and spark plug being provided for each cylinder in a multi-cylinder system.
To stop an engine equipped with a capacitor discharge ignition system, it is necessary to short out the power storage capacitor and thus prevent operation of the spark plug. A grounding terminal is normally provided so that a switch may be connected to the ignition system to shut down the engine. This grounding terminal is capable of providing sufficient energy to cause ignition of a hazardous atmosphere. Therefore, the switch, or switches, must be enclosed in an explosion proof enclosure suitable for Class 1, Division 1, Group D hazardous locations.
If the grounding circuit could be converted to a nonincendive, that is, not capable of causing ignition, the National Electrical Code allows a switch enclosed in a housing for ordinary locations to be used. This would reduce costs, space requirements, and would be safer in that it would not depend on maintenance of explosion proof enclosures for safety.
For many years, suppliers have provided control systems that operate from the voltage available at the grounding terminal of engine ignition systems. These systems consist of pressure, temperature and level operated mechanical switches and manually re-set electrically tripped indicating relays. Because some capacitor discharge ignition systems do not supply sufficient energy to reliably trip the indicating relays, an auxiliary power storage capacitor has been supplied. The output of this capacitor is, of course, ignition capable in a hazardous location and requires explosion proof enclosures which carry the same penalty as the explosion proof enclosures used for the shorting switches as previously outlined.